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The concept of a foil hat for protection against interference of the mind was mentioned in a science fiction short story by Julian Huxley, "The Tissue-Culture King", first published in 1927, in which the protagonist discovers that "caps of metal foil" can block the effects of telepathy.[2]

While there is no evidence that anyone has ever in seriousness advocated wearing a literal tinfoil hat, headgear intended to block telepathy is advocated.[3] This is often based on a belief that such hats prevent mind control by governments, spies, or paranormal beings that employ ESP or the microwave auditory effect. People in many countries who believe they are "Targeted individuals" (TIs), subject to government spying or harassment, have developed websites, conference calls, and support meetings to discuss their concerns, including the idea of protective headgear.[4] Over time the term "tin foil hat" has become associated with paranoia and conspiracy theories.[5]

The notion that a metal foil hat can significantly reduce the intensity of incident radio frequency radiation on the wearer's brain has some scientific validity, as the effect of strong radio waves has been documented for quite some time.[6] A well-constructed aluminum foil enclosure would approximate a Faraday cage, reducing the amount of (typically harmless) radiofrequency electromagnetic radiation passing through to the interior of the structure. A common high school physics demonstration involves placing an AM radio on aluminum foil, and then covering the radio with a metal bucket. This leads to a noticeable reduction in signal strength. The efficiency of such an enclosure in blocking such radiation depends on the thickness of the foil, as dictated by the skin depth, the distance the radiation can propagate in a particular non-ideal conductor. For half-millimetre-thick aluminum foil, radiation above about 20 kHz (i.e., including both AM and FM bands) would be partially blocked, although aluminum foil is not sold in this thickness, so numerous layers of foil would be required to achieve this effect.[7]

The effectiveness of the aluminum foil hat as electromagnetic shielding for stopping radio waves is greatly reduced by it not being a complete enclosure. Placing an AM radio under a metal bucket without a conductive layer underneath demonstrates the relative ineffectiveness of such a setup. Indeed, because the effect of an ungrounded Faraday cage is to partially reflect the incident radiation, a radio wave that is incident on the inner surface of the hat (i.e., coming from underneath the hat-wearer) would be reflected and partially 'focused' towards the user's brain.

A study by graduate students at MIT determined that an aluminum foil hat could amplify or attenuate incoming radiation depending on frequency. At cellular phone frequencies – 2.6 GHz is amplified by 30 dB. At 1.2 GHz, a 20 dB amplification was observed. However, at 1.5 GHz, a 20 dB attenuation was observed, with no significant attenuation beyond 10 dB anywhere else.[8] At some microwave wavelengths, the skin depth is less than the thickness of even the thinnest foil.[9][relevant?– discuss]

A belief also exists that aluminum foil is a protective measure against the effects of electromagnetic radiation (EMR). Despite some allegations that EMR exposure has negative health consequences,[10] no link has been established between the radio-frequency EMR that foil hats are meant to protect against, and subsequent ill health.[citation needed]

^Huxley, Julian (1927). The Tissue-Culture King. Well, we had discovered that metal was relatively impervious to the telepathic effect, and had prepared for ourselves a sort of tin pulpit, behind which we could stand while conducting experiments. This, combined with caps of metal foil, enormously reduced the effects on ourselves.